Communication system

ABSTRACT

A method for reducing ringing in a signal output from a filter comprising inputting a signal into a filter; filtering a first portion of the input signal to generate a filtered portion of the output signal; analyzing the filtered portion of the output signal; detecting if ringing is present in the filtered portion of the output signal based on said analysis; and adjusting the filter characteristics to reduce ringing in a subsequent filtered portion of the output signal if it is determined that ringing is present.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 or 365 to GreatBritain, Application No. 0704732.7, filed Mar. 12, 2007. The entireteachings of the above application are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to signal processing. More particularlythe present invention relates to a method and apparatus for removingringing in a signal.

BACKGROUND

In a communication system a communication network is provided, which canlink together two communication terminals so that the terminals can sendinformation to each other in a call or other communication event.Information may include speech, text, images or video.

Modern communication systems are based on the transmission of digitalsignals. Analogue information such as speech is input from a microphoneinto an analogue to digital converter at the transmitter of one terminaland converted into a digital signal. The digital signal is then encodedand placed in data packets for transmission over a channel to thereceiver of a destination terminal. When the digital signal is receivedat the destination terminal, the signal is decoded before it is inputinto a digital to analogue converter. The digital to analogue converteroutputs an analogue signal to a loudspeaker or other output interface.

Analogue signals are often contaminated by electromagnetic interferencefrom the power grid. In microphone signals, for example, this isperceived as a steady “hum”. Depending on geographical location, thishum consists of 50 or 60 Hertz signals plus harmonics, with energytypically concentrated in the lowest few harmonics. Removing theseinterfering harmonics can greatly improve the perceived quality of themicrophone signal.

Traditional narrowband speech encoding algorithms encode signals withina frequency range of about 300-3400 Hz. By removing the signalinformation below 300 Hz, the lowest, and usually strongest, power gridharmonics are already filtered out, which significantly reduces theproblem of power grid interference.

However recently there has been a demand for speech encoders that haveimproved quality and provide a natural sounding speech output. This hasled to the development of wideband speech encoding methods such asAMR-WB (Adaptive Multi Rate-Wide Band) which encode frequencies from50-7000 Hz. For wideband speech encoding methods, the lowest power gridharmonics fall within the coded frequency band. Consequently whenemploying encoding techniques that encode lower frequencies theperceived hum caused by the power grid harmonics is more severe.

Since the frequency of the power grid signal is very stable, most knownmethods employ notch filters which remove a fixed frequency band toprovide a simple and effective method for removing these interferingsignals. The width of the frequency band removed by the notch filter mayotherwise be referred to as the width of the notch filter. For example anotch filter with a frequency response that removes a narrow frequencyband is referred to as a narrow width notch filter.

FIG. 1 shows an implementation of a notch filter. The filter comprisessingle sample delay elements 10 and 12, gain elements 14 and 16 andmixer elements 18, 20, 22 and 24. A signal s(n) is input into the filterand a signal y(n) is output from the filter.

In the time domain, the relationship between the output signal y(n) andthe input signal s(n) may be expressed as:

y(n)=s(n)−Bs(n−1)+s(n−2)+(B−A)y(n−1)−(1−A)y(n−2)   Equation (1)

Where A and B are the gain coefficients of gain elements 16 and 14respectively.

A function referred to as the Z transform, commonly used in the field ofsignal processing, may be used to convert a discrete time domain signalinto a frequency domain representation.

The Z transform of a time signal x(n) may be defined as:

$\begin{matrix}{{X(z)} = {{X\left\{ {x(n)} \right\}} = {\sum\limits_{n = 0}^{\infty}\; {{x(n)}z^{- n}}}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

Where z is a complex number:

z=Ke^(jφ)  Equation (3)

where K is the magnitude of z, and φ is the angular frequency.

The Z transform Y(z) of the time domain output signal y(n) and the Ztransform S(z) of the input signal s(n) may be used to show the transferfunction H(z) of the filter:

$\begin{matrix}{{H(z)} = {\frac{Y(z)}{S(z)} = \frac{1 - {B\; z^{- 1}} + z^{- 2}}{1 - {\left( {B - A} \right)z^{- 1}} + {\left( {1 - A} \right)z^{- 2}}}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

FIG. 2 shows the frequency response of the notch filter shown in FIG. 1.In particular FIG. 2 shows the frequency response of the filter in thelowest part of the spectrum, from 0 Hz to 100 Hz. As shown, the region Rin the graph referred to hereinafter as the notch region represents thefrequencies that are removed by the notch filter. The width of the notchregion corresponds to the width of the frequency band removed by thenotch filter, and hence the width of the notch filter.

The gain coefficient A determines the width of the notch region and thegain coefficient B determines the frequency at the centre of the notchregion, hereinafter referred to as the centre frequency F_(C). Thus, thewidth of the notch filter may be set by selecting the value of the gaincoefficient A of the first gain element 16. Similarly, the centre valueof the frequency removed by the notch filter may be set by selecting thevalue of the gain coefficient B of the second gain element 14.

More specifically the notch width at −3 dB is related to the coefficientA by:

W_(3 dB)≠0.16 F_(S)A   Equation (5)

where W_(3 dB) is the notch width in Hertz (Hz), between the two slopesof the notch region R at −3 dB, and F_(S) is the sampling frequency inHz. It can therefore be seen that, for a narrow notch filter, thecoefficient A is made to be slightly larger than zero.

The notch centre frequency is related to the coefficient B by:

F _(C)≠0.16 F _(S) arccos(B/2)   Equation (6)

where F_(C) is the centre frequency in Hz, F_(S) is the samplingfrequency in Hz, and arccos is an arc cosine mapping. Setting thecoefficient B of the second gain element 14 to a value slightly lowerthan 2 the notch filter can be configured to have a centre frequencyF_(C) of 50 Hz.

To avoid removing significant parts of the desired signal, the width ofsuch notch filters must be very narrow. Using the coefficients;

-   A=0.001-   B=1.9996137    the notch filter can is configured to have a notch centred at 50 Hz    and with a width W_(3 dB) of 2.6 Hz, for a sampling frequency F_(S)    of 16 kHz.

However the use of narrow notch filters leads to an undesired phenomenonknown as ringing. Ringing is an exponentially damped sinusoidal responseof a filter to an impulse. For example, if the input signal is speech,the notch filter may occasionally be excited by harmonics of the speechsignal matching the notch frequency. After the burst of speech, thenotch filter will continue to produce a damped sinusoid at the notchfrequency for perhaps a few seconds, depending on the notch width. Thisringing gives an undesired reverberation-like quality to the sound. Theringing time of a notch filter increases in inverse proportion to thenotch width in frequency. Therefore, for narrow filters the ringing timeis particularly pronounced.

The time response of the notch filter shown in FIG. 1 is depicted inFIG. 3, which shows significant ringing for about half a second. FIG. 3assumes a nonzero filter state, that is, the time delay elements 10 and12 of the filter have a residual energy caused by a previous nonzeroinput. A similar time response would be achieved in response to animpulse input into a filter having a zero filter state, having noresidual energy from a previous input. This is known as the impulseresponse of the filter.

A notch filter that is sufficiently narrow not to remove audibleportions of the signal will introduce audible ringing in the signal.Conversely if the width of the notch filter is designed to be wideenough not to cause ringing in the signal the filter will audibly removeparts of the input speech signal.

An alternative method for removing steady sinusoidal components from aninput signal employs adaptive cancellation, see e.g., H. C. So,“Adaptive Algorithm for Sinusoidal Interference Cancellation,”Electronics Letters, vol. 33, no. 22, pp. 1910-1912, October 1997.Adaptive cancellation works in three steps. First, the parameters of oneor more interfering sinusoids are adaptively estimated. Then sinusoidsare synthesized based on those parameters. Finally, the synthesizedsinusoids are subtracted from the input signal to create the hum-removedsignal. In case of power grid harmonics, the frequency parameters areknown and essentially constant, so that only the amplitude and phaseparameters need to be estimated. These parameters can be estimatedadaptively using any well-known adaptation method such as the Least MeanSquares (LMS) algorithm.

Adaptive cancellation suffers from very much the same problems as notchfiltering, in that a trade off is made between ringing and distortionsof the desired component of the input signal. Distortions may result ifthe parameter estimation adapts too quickly, whereas ringing may occurwhen the parameter estimation adapts too slowly.

It is therefore an aim of the present invention to overcome the problemspresented by the prior art.

SUMMARY

According to a first aspect of the present invention there is provided amethod for reducing ringing in a signal output from a filter comprising:inputting a signal into a filter; filtering a first portion of the inputsignal to generate a filtered portion of the output signal; analyzingthe filtered portion of the output signal; detecting if ringing ispresent in the filtered portion of the output signal based on saidanalysis; and adjusting the filter characteristics to reduce ringing ina subsequent filtered portion of the output signal if it is determinedthat ringing is present.

According to a second aspect of the present invention there is provideda device arranged to reduce ringing in an output signal comprising: areceiver arranged to receive an input signal; a filter arranged tofilter a first portion of the input signal to generate a filteredportion of the output signal; and a controller arranged to analyze thefiltered portion of the output signal; detect if ringing is present inthe filtered portion of the output signal based on said analysis; andadjust the filter characteristics to reduce ringing in a subsequentfiltered portion of the output signal if it is determined that ringingis present.

One advantage of dynamically adjusting the notch filter impulse responsein accordance with embodiments of the present invention is that itallows for the use of notch filters to effectively remove an undesiredsignal component of steady frequency, such as a power grid harmonic,without causing the removal of substantial parts of the signal whileeffectively preventing ringing in the output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried into effect, embodiments of the present inventionwill now be described with reference to the following drawings:

FIG. 1 shows a notch filter in accordance with the prior art;

FIG. 2 shows the frequency response of the notch filter shown in FIG. 1;

FIG. 3 shows the time response of the notch filter shown in FIG. 1;

FIG. 4 shows an adjustable notch filter in accordance with an embodimentof the present invention;

FIG. 5 shows an adjustable notch filter in accordance with an furtherembodiment of the present invention;

FIG. 6 is a graph showing a damped frequency response of a notch filterin accordance with an embodiment of the present invention;

FIG. 7 is a graph showing a damped time response of a notch filter inaccordance with an embodiment of the present invention;

FIG. 8 shows a circuit for controlling the adjustable notch filter inaccordance with an embodiment of the present invention;

FIG. 9 which shows a series of adjustable filters used to removemultiple harmonics, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

The inventor of the present invention has found that the problemspresented in the prior art may be overcome by adjusting the impulseresponse of a notch filter in accordance with the characteristics of thesignal output from the filter. More specifically, the impulse responseof the notch filter may be dynamically adjusted to reduce ringing whenringing is detected on the signal output from the filter. By adjustingthe impulse response of the notch filter to reduce ringing when ringingis present on the output signal, significantly reduces the audibility ofringing in the output signal without removing significant parts of thesignal when no ringing is present.

In accordance with an embodiment of the invention damping may be appliedto the impulse response of the filter when ringing is detected. This maybe achieved by adjusting the frequency response of the filter independence on detecting ringing in the output signal.

According to one embodiment of the invention the frequency response ofthe filter may be adjusted such that the width of the notch region isincreased during ringing, thereby damping the impulse response of thenotch filter and reducing ringing. Alternatively, or additionally thefrequency response of the filter may be adjusted such that the depth ofthe notch region is decreased during ringing, thereby damping theimpulse response of the notch filter and reducing ringing.

By increasing the width of the notch region and decreasing the depth ofthe notch region the output signal is made to follow the input signalmore closely, thus damping the impulse response of the notch filter.

The manner in which an embodiment of the present invention may beimplemented will first be described with reference to FIG. 4. FIG. 4shows an adjustable notch filter 26 having an adjustable notch width.The components that are described in relation to the fixed notch filtershown in FIG. 1 are referred to in FIG. 4 using like reference numerals.

In FIG. 4 the first gain element 16 shown in FIG. 1 is replaced by anadjustable gain element 16′. The width of the notch filter 26 may beadjusted by adjusting the gain coefficient A of the adjustable gainelement 16′. The mixer element 24 is arranged to subtract the output ofthe adjustable gain element 16′ from the input signal s(n). Byincreasing the coefficient A, the width of the frequency band applied tothe mixer element 24 is increased. According to an embodiment of theinvention the gain coefficient A is adjusted to increase the width ofthe notch filter when ringing is detected on the output signal y(n).Increasing the width of the notch filter dampens the impulse response ofthe filter 26, thus reducing the ringing time.

Increasing the coefficient A to reduce ringing in the output signal y(n)may however cause artifacts in the signal output from the adjustablegain element 16′. In a preferred embodiment of the invention anadjustable notch filter as shown in FIG. 5 is used to filter the signal.The adjustable notch filter 26′ as shown in FIG. 5 is adjustable innotch width and depth, and is arranged to produce less artifacts thanthe filter shown in FIG. 4. It will be appreciated by a person skilledin the art that different possible filter arrangements will producedifferent artifacts in response to changing filter coefficients.

The components that are described in relation to the fixed notch filtershown in FIG. 1 are referred to in FIG. 5 using like reference numerals.In addition to the components included in the fixed notch filterdescribed in relation to FIG. 1 the adjustable notch filter 26′ alsoincludes a third gain element 28 which applies a gain G. The third gainelement 28 is adjustable.

As shown in FIG. 5 the first mixer element 22 adds together the outputsignal y(n) with the output of the second mixer 20 and subtracts fromthis the output of the third gain element 28 to provide an output to thefirst single sample delay element 10. The delay element 10 introduces adelay to the signal output from the first mixer 22 before outputting thesignal to the first gain element 14, the second delay element 12 and thethird mixer 18. The first gain element applies a gain B to the signalreceived from the delay element 10.

The second mixer 20 subtracts the output of the second delay element 12from the output of the first gain element and outputs the resultingsignal to the first mixer element 22.

The third mixer 18 subtracts the output of the second delay element 12from the output of the first delay element 10 and outputs the resultingsignal to the second gain element 16 where a gain A is applied. Theoutput of the second gain element 16 is applied to the fourth mixer 24where it is subtracted from the input signal s(n). The output of thesecond gain element 16 is also input into the third gain element 28where a gain G is applied to the signal.

For the filter shown in FIG. 5, the relationship between the inputsignal s(n) and the output signal y(n) can be expressed as:

y(n)=s(n)−(B−GA)s(n−1)+(1−GA)s(n−2)+(B−(G+1)A)y(n−1)−(1−(G+1)A)y(n−2)  Equation (7)

Using the z transform S(z) of the input signal and the z transform ofthe output signal Y(z) the adjustable notch filter shown in FIG. 5 canbe shown to have a transfer function:

$\begin{matrix}{{H(z)} = {\frac{Y(z)}{S(z)} = \frac{1 - {\left( {B - {G\; A}} \right)\; z^{- 1}} + {\left( {1 - {G\; A}} \right)z^{- 2}}}{1 - {\left( {B - {\left( {G + 1} \right)A}} \right)z^{- 1}} + {\left( {1 - {\left( {G + 1} \right)A}} \right)z^{- 2}}}}} & {{Equation}\mspace{14mu} (8)}\end{matrix}$

Where A is the gain coefficient of the second gain element 16, B is thegain coefficient of the first gain element 14 and G is an adjustablegain coefficient of the third gain element 28.

If the gain coefficient G of the third gain element 28 is made equal tozero, then the frequency response of the adjustable notch filter shownin FIG. 5 will be the same as the frequency response of the notch filtershown in FIG. 1. However, by making the gain coefficient G larger thanzero, without changing the values of the gain coefficients A and B thefrequency response of the filter changes such that the width of thenotch filter increases and the notch becomes shallower. As a result theimpulse response of the filter is dampened.

FIG. 6 shows a graph of the frequency response of the adjustable filtershown in FIG. 5 when G=10. When the graph shown in FIG. 6 is compared tothe frequency response graph shown in FIG. 2 it can be seen that thenotch region R shown in FIG. 5 is wider and shallower than the notchregion shown in FIG. 2.

FIG. 7 shows a graph of the time response of the adjustable filter shownin FIG. 5 when the coefficient G is made equal to 10. Comparing the timeresponse shown in FIG. 7 to the time response shown in FIG. 3 it can beseen that the ringing time as shown in FIG. 7 is significantly reduced,in this case to approximately 50 milliseconds.

In one embodiment of the invention when no ringing is present in theoutput signal the adjustable notch filter is used such that G is set to0, and the notch filter will remove any steady tonal component at thefrequency to which it is tuned. Only when ringing is present will theadjustable coefficient G be increased to dampen the notch filter so thatthe ringing will die out quickly, after which G is returned to 0.

The inventor of the present invention has identified that duringringing, at some instances the output energy will be larger than theinput energy. Accordingly, the inventor of the present invention hasfound that ringing in the output signal may be detected if the magnitudeof the output energy is determined to be greater than the input energyat a given instance.

According to a preferred embodiment of the invention, ringing in theoutput signal y(n) may be detected by detecting that the magnitude ofthe energy of the output signal is greater than the magnitude of theenergy of the input signal.

FIG. 8 shows a circuit for controlling the adjustable notch filter 26 inaccordance with an embodiment of the present invention. The controlcircuit shown in FIG. 8 includes the adjustable notch filter 26, acontroller block 34, an input energy measure block 32 and an outputenergy measure block 30.

The adjustable notch filter is arranged to receive an input signal s(n)and to output a signal y(n). The input signal s(n) is also input intothe input energy measure block 32. The output signal is input into theoutput energy measure block 30. The controller block 34 receives aninput from the input energy measure block 32 and the output energymeasure block 30. The adjustable notch filter 26 receives a controlsignal from the controller block 34.

The input and output signals may be regarded as comprising a series offrames, each frame containing an equal portion of the signal. Forexample each frame may have a frame length of 10 milliseconds. Inoperation the input energy measure block 32 determines the energy foreach frame of the input signal. In the same manner, the output energymeasure block 34 determines the energy for each frame of the outputsignal y(n). The energy determined for each frame of the input signaland the output signal is reported to the controller block 34.

The controller block 34 is arranged to compare the energy determined forthe input signal s(n) and the energy determined for the output signaly(n) for each frame. When the magnitude of the energy of a frame of theoutput signal exceeds the magnitude of the energy of a frame of theinput signal, the controller is arranged to output a control signal tothe adjustable notch filter 26 to dampen the time response of the filter26. In a preferred embodiment of the invention the time response of thefilter 26 is damped by increasing the width of the notch filter anddecreasing the depth of the notch region. When employing the adjustablenotch filter described with reference to FIG. 5, this may be achieved byincreasing the gain coefficient G of the third gain block 28 from 0 to10. The other coefficients A and B are kept unchanged.

As soon as the magnitude of the energy of a subsequent frame of theoutput signal y(n) is less than the magnitude of the energy of the inputsignal s(n), the G parameter is reset to 0, thus narrowing the notch ofthe filter 26.

Reference is now made FIG. 9 which shows the transmitting circuitry 48of a terminal in accordance with an embodiment of the invention. Inorder to remove multiple harmonics, the input signal s(n) is processedthrough a series of adjustable notch filters, each tuned to a differentharmonic frequency.

The transmitting circuitry 48 includes a series of adjustable filtersused to remove multiple harmonics. As shown in FIG. 9, four adjustablenotch filters 40, 42, 44 and 46 are provided having a notch tuned to 50Hz, 60 Hz, 100 Hz and 120 Hz respectively. More or fewer adjustablenotch filters can be used to remove more or fewer harmonics.

The transmitting circuitry also includes a microphone 50, an analogue todigital converter 52, a high pass filter 54, a speech encoder 56, aninput energy measure block 32, an output energy measure block 30 and acontroller 34.

The microphone is arranged to receive an input signal to be transmittedto a destination terminal via a communication network. The input signalmay comprise speech input by a user of the terminal. The microphoneoutputs the analogue input signal to the analogue to digital converter52. The analogue to digital converter converts the analogue input signalto a digital input signal s(n). The digital input signal is then inputinto a high pass filter 54 which removes the low frequencies from theinput signal s(n).

The high pass filter outputs the signal s(n) to the series of adjustablenotch filters. The signal s(n) is also input into the input energymeasure block 32. The signal y(n) output from the series of adjustablenotch filters is input into an encoder 56 that is used to encode thesignal y(n) before transmitting the signal. The signal y(n) is alsooutput from the series of adjustable notch filters to the output energymeasure block 30.

The input energy measure block 32 and the output energy measure block 30report the magnitude of the energy of the input and output signalsrespectively to the controller block as described in relation to FIG. 8.If the controller block determines that the magnitude of the energy of aframe of the output signal exceeds the magnitude of the energy of aframe of the input signal, the controller is arranged to output acontrol signal to each adjustable notch filter to dampen the impulseresponse of each filter.

As soon as the magnitude of the energy of a subsequent frame of theoutput signal y(n) is less than the magnitude of the energy of the inputsignal s(n), the damping applied to the impulse response of the filteris removed.

According to an alternative embodiment of the invention, a separatecontrol block may be used to control the width of each adjustable notchfilter provided in the transmitting circuitry. According to thisembodiment of the invention an input energy measure block and an outputenergy measure block are provided for each filter to measure the energyof the input signal and the output signal of each filter. The width ofeach filter is then controlled independently of the other filters in theseries.

According to a further alternative embodiment of the invention theimpulse response of the filter may be damped by reducing the depth ofnotch region of the adjustable notch filter, without necessarilyincreasing the width of the filter. For the filter shown in FIG. 5, thedepth of the notch region of the filter may be varied independently ofthe width of the notch region by adjusting the gain coefficient A of thegain element 16 to effectively cancel notch width variations caused bychanging the gain coefficient of the gain element 28.

The above described embodiments may be implemented as hardware in aterminal or as software running on a processor in a terminal. Thesoftware for providing the operation may be stored on and provided bymeans of a carrier medium such as a carrier disc, card or tape. Apossibility is to download the software via a data network. This is animplementation issue.

While this invention has been particularly shown and described withreference to preferred embodiments, it will be understood to thoseskilled in the art that various changes in form and detail may be madewithout departing from the scope of the invention as defined by theclaims.

1. A method for reducing ringing in a signal output from a filtercomprising: inputting a signal into a filter; filtering a first portionof the input signal to generate a filtered portion of the output signal;analyzing the filtered portion of the output signal; detecting ifringing is present in the filtered portion of the output signal based onsaid analysis; and adjusting the filter characteristics to reduceringing in a subsequent filtered portion of the output signal if it isdetermined that ringing is present.
 2. A method as claimed in claim 1wherein adjusting the filter characteristics allows the output signal tosubstantially follow the input signal.
 3. A method as claimed in claim 1wherein the portion of the input signal is a frame of the input signal.4. A method as claimed in claim 1 wherein the portion of the outputsignal is a frame of the output signal.
 5. A method as claimed in claim1 wherein the filter characteristics are adjusted by damping the impulseresponse of the filter.
 6. A method as claimed in claim 1 wherein thefilter is a notch filter.
 7. A method as claimed in claim 6 wherein thefilter characteristics are adjusted by increasing the width of the notchfilter.
 8. A method as claimed in claim 6 wherein the filtercharacteristics are adjusted by decreasing the depth of the notchfilter.
 9. A method as claimed in claim 6 wherein the notch filterincludes at least one gain element.
 10. A method as claimed in claim 9wherein the width of the notch filter is increased by adjusting a gaincoefficient of said at least one gain element.
 11. A method as claimedin claim 9 wherein the depth of the notch filter is decreased byadjusting a gain coefficient of said at least one gain element.
 12. Amethod as claimed in claim 1 further comprising the steps of: analyzingthe signal subsequent filtered portion of the output signal; detectingif ringing is present on the subsequent filtered portion of the outputsignal based on said analysis; and adjusting the filteringcharacteristics with an equal and opposite adjustment to the adjustmentthat was made to remove ringing if it is determined that ringing is notpresent.
 13. A method as claimed in claim 1 wherein said step ofanalyzing the filtered portion of the output signal comprises;determining the energy of a frame of the input signal; and determiningthe energy of a frame of the filtered portion of the output signal; andwherein said step of detecting if ringing is present on the filteredportion of the output signal comprises; comparing the energy of theframe of the filtered portion of the output signal to the energy of theframe of the input signal; and determining that ringing is present onthe output signal if the energy of the frame of the filtered portion ofthe output signal is greater than the energy of the frame of the inputsignal.
 14. A device arranged to reduce ringing in an output signalcomprising: a receiver arranged to receive an input signal; a filterarranged to filter a first portion of the input signal to generate afiltered portion of the output signal; and a controller arranged toanalyze the filtered portion of the output signal; detect if ringing ispresent in the filtered portion of the output signal based on saidanalysis; and adjust the filter characteristics to reduce ringing in asubsequent filtered portion of the output signal if it is determinedthat ringing is present.
 15. A computer program comprising program codemeans adapted to perform the steps of claim 1 when the program is run ona processor.